74 COJ^KLIN— EMBRYOLOGY OF A BRACHIOPOD. [April 4. 



Fig. 17. Oral view of embryo, showing elongated blastopore opening at its 

 anterior end into the enterocoel and enteron; the enterocoel but little larger 

 than the enteron. 



Fig. 18. Antero- ventral view of an embryo of about the same stage as the 

 preceding. 



Plate IV. 



Fig. 19. Optical section of older embryo, viewed from posterior; showing the 

 enteron separated from the enterocoel. 



Fig. 20. Optical section of an embryo of same stage as preceding, lateral 

 view ; showing the enteron almost entirely separated from the enterocoel. 



Fig. 21. Oral view of an embryo of about the same stage as the preceding; the 

 blastopore narrower than in preceding stages ; the enteron completely 

 cut off from the enterocoel except in the region of the blastopore ; mesen- 

 chyme cells {mc) line the anterior part of the enterocoel. 



Fig. 22. Oral view of an older embryo in which the blastopore has closed to a 

 narrow groove except for a small opening near its anterior end ; mesenchyme 

 cells are abundant in the anterior and posterior parts of the enterocoel ; man- 

 tie folds show at the sides of the embryo. 



Fig. 23. Aboral view of an embryo of the same stage as the preceding, showing 

 dorsal mantle fold {M). 



Fig. 24. Optical longitudinal section of an embryo in which the blastopore has 

 completely closed, leaving however on the ventral surface the blastopore 

 remnant {Br) and groove; the dorsal mantle fold {Md) and furrow are 

 shown on the dorsal side ; the enteron still communicates with the enterocoel 

 at its posterior end. 



Plate V. 



Fig. 25. Dorsal view of a larva in which cephalic, mantle and peduncular 

 regions are well defined. 



Fig. 36. Ventral view of an older larva, showing the ventral mantle folds 

 meeting m the region of the blastopore groove [^Bf); the blastopore rem- 

 nant {Br) lies in a notch on the anterior side of the mantle fold. 



Fig. 27. Dorsal view of a slightly older larva, showing the increased promi- 

 nence of the mantle and the lobulation of the coelom. 



Fig. 28. Optical longitudinal section of a larva of the same stage as the pre- 

 ceding, showing the dorsal and ventral mantle folds {Md axidi. AIv) ; apical 

 sense plate and ganglion ; enteron and coelom. 



Fig. 29. Ventral view of an older larva, showing the ventral mantle folds fused 

 in the midline ; the anterior and posterior mantle furrows are shown as 

 shaded lines ; in the anterior mantle furrow the place at which the blastopore 

 remnant disappeared and where the oesophageal invagination will occur is 

 marked ( O) ; the apical and ventral sense plates ( CG and SG). 



